Previous research under the Southern Oxidant Study (SOS) had shown that a predominance of ozone design days for urban areas occurred under conditions where synoptic stationary fronts or analyzed trough lines (often representing old air mass boundaries) were present in the area. In an analysis of synoptic structures associated with high one hour and eight hour ozone in the Dallas Ft. Worth (DFW) area it was also found that stationary fronts were associated with the three highest ozone events in the summer of 1999.

A preliminary analysis of these events examining the synoptic structure and air quality structure found:

1.      The deformed flows characteristic of synoptic stationary fronts give rise to narrow zones of light winds without vertical lifting. The result is an area of reduced dilution that if located over even modest emission source regions could produce high precursor and ozone concentrations.

2.      Trajectories into DFW under these events showed relatively short trajectories over 24-48 hours indicating local influences rather than long range transport. However, the longer trajectories showed considerable descent from the mid-troposphere to the surface.

3.      Considerable moist deep convection (thunderstorms) was present in both episodes.

As a result of this study and the previous findings under SOS a pilot program was developed and proposed to use aircraft during the TEXAQSII to sample in the vicinity of a stationary front if one presented itself. The Baylor Aztec aircraft with ozone, NOy, backscatter along with winds, temperature and pressure was selected to fly the stationary front experiment. On August 21 a stationary front was forecasted to sag near to the DFW area. A flight was planned and carried out to sample across the front and the vertical structure. A similar flight was carried out the following day when the front sagged south of DFW. Details of the synoptic situation are provided along with preliminary MM5 model simulations and Lagrangian trajectories.  The details of the flights including ozone and NOy distributions are provided along with vertical profiles. In general the data showed that highest ozone occurred in the vicinity of the front. There was no indication that ozone came from aloft near the front in that highest concentrations were near the ground. South of the front at the start of the flight the highest ozone was aloft. However, this could be due to ozone losses near the surface. Also, because of the limited height of the Aztec measurements it was not clear that true free troposphere air north of the front was well sampled.